1. Field of the Invention
The present invention relates to a distillation apparatus for performing distillation of a cryogenic fluid, in which a plurality of distillation columns is connected in the form of a cascade. In particular, the present invention relates to a distillation apparatus which is suitable for enrichment of stable isotopes (13C, 15N, 17O, 18O, etc.) of carbon, nitrogen, oxygen, etc. which exist in only little amounts in nature.
2. Description of Related Art
Stable isotopes (13C, 15N, 17O, 18O, etc.) of carbon, nitrogen, oxygen, etc. are used as a tracer in the fields of natural science and medical care. As an enrichment method of these isotopes which exist in only little amounts in nature, there is a cascade process which uses a plurality of distillation columns.
Cascade means connecting a plurality of distillation columns in series. In order to continuously concentrate a specified component in raw materials, the component concentrated in a distillation column is further concentrated in a latter distillation column, and again concentrated in a further latter distillation column. That is, a continuous distillation process is performed by a plurality of divided distillation columns. In this respect, a cascade process is different from a process seen in a general chemical process, which combines a plurality of distillation columns in which a component to be concentrated is different from the others.
A cascade process is a technique which is mainly used in the field of isotope enrichment. This cascade process enables enrichment by distillation for a structural isomer or an isotope which has a separation factor (also referred to as relative volatility) of almost 1, requires a very large number of theoretical plates, and is difficult to be separated.
Hereinafter, an example of a conventional cascade process is described. In a cascade process, as a method of exchanging a material between adjacent distillation columns, i.e. a connection method, there are methods shown in FIG. 2 to FIG. 5 (see non-patent references 1 to 3).
The distillation apparatus shown in FIG. 2 is an example of the simplest distillation cascade. This example of a distillation apparatus is composed of a distillation column group in which 6 distillation columns D1 to D6 are connected in series. The distillation columns D1 to D3 constitute a recovery part, and the distillation columns D3 to D6 constitute an enrichment part. In the distillation columns D1 and D2, the condensers C1 and C2 are provided on the tops thereof. In the distillation column D4 to D6, the reboilers R4 to R6 are provided on the bottoms thereof. In the distillation columns D3, the condenser C3 is provided on the top thereof, and the reboiler R3 is provided on the bottom thereof.
A feed gas F is fed into the distillation column D3. Then, the desired component is concentrated and withdrawn from the bottom part of the distillation column D6 as a product P, and the others are withdrawn from the top part of distillation column D1 as waste components W.
In the distillation column D3 to which the feed gas F is fed, the distillation load is the largest. The load becomes gradually small toward the last column D6 in the enrichment part and toward the first column D1 in the recovery part (that is, the column diameter becomes small).
In the example of this apparatus, the returns of gases from the last column D6 to the fifth column D5, from the fifth column D5 to the fourth column D4, . . . , and from the second column D2 to the first column D1 are performed by using pressure differences. Therefore, the pressure of a distillation column needs to be high toward the last column D6 from the first column D1. As a result, a separation factor (=relative volatility) also becomes small, thereby resulting in a disadvantage with respect to distillation efficiency.
In this case, when liquid pumps P1 to P5 are used to flow the liquid of a distillation column to the latter column, liquid is accumulated in the pumps. Therefore, the liquid hold-up over the whole apparatus is increased. This results in the disadvantage that startup time becomes long.
Also, in the case of cryogenic distillation, the use of a liquid pump causes an increase in heat inleak, thereby resulting in a disadvantage in this respect.
The distillation apparatus shown in FIG. 3 is another example of conventional art. The distillation columns thereof have the same features as the example of the apparatus shown in FIG. 2. In this example, pressures at the tops are decreased, and pressures at the tops of all the distillation columns D1 to D6 are the same. This apparatus can prevent a pressure from increasing toward the last column D6, but requires a pressurizing device such as blowers B1 to B5, resulting in a disadvantage with respect to reliability of the apparatus. Also, the disadvantages regarding the use of the liquid pumps P1 to P5 are not solved.
The example shown in FIG. 4 is an example of conventional art which is the developed version of the apparatus shown in FIG. 3. In a similar manner to the apparatus shown in FIG. 3, pressures at tops of all distillation columns D1 to D6 are decreased. On all the distillation columns D1 to D6, the condensers C1 to C6 and the reboilers R1 to R6 are provided, and a gas is fed into a latter column by the pressure difference between distillation columns (corresponding to pressure loss in the case where pressures at tops are the same).
In this apparatus, because liquid pumps are not used, it is possible to decrease liquid hold-up. However, the disadvantage in that the blowers B1 to B5 are required to return a gas is not solved.
The distillation apparatus shown in FIG. 5 is a modified example of the apparatus shown in FIG. 4. In this example, blowers used for returning is omitted, and instead, the liquid obtained by liquefaction in condensers C1 to C6 is stored in liquid-return lines Q1 to Q5 so as to return this liquid to a former column by the liquid head pressure (liquid head) therein.
In this apparatus, both the feeding and returning devices do not require a rotary machine such as a pump or a blower, reliability of this apparatus is improved, and liquid hold-up in a liquid-return line can be minimized, thereby resulting in an advantage with respect to reduction of startup time. Also, there is an advantage that pressures of all the distillation columns D1 to D6 are low because it contributes to increase a separation factor. However, a condenser and a reboiler are required for every distillation column, resulting in a disadvantage with respect to cost of the apparatus.
In a cascade process which is used in enrichment of a structural isomer or an isotope, the full length of a distillation column becomes very long, and the amount of liquid hold-up is increased necessarily. Also, the separation factor is small. Therefore, the problem in that startup time becomes long occurs.
As a result, in the aforementioned cascade method, the distillation apparatus shown in FIG. 5 can be the best because the separation factor is larger than those of the other distillation apparatus, there is not a lot of liquid hold-up, and a rotary machine is not required for each distillation column.
However, the distillation apparatus shown in FIG. 5 also has disadvantages. For example, there is the following disadvantage. In a process such as cryogenic separation for O2, CO, NO, CH4, N2, etc., which is operated at a lower temperature than ambient temperature, a part of the liquid stored in a liquid-return line is evaporated by heat inleak, and so a liquid is difficult to be stored in a liquid-return line.
FIG. 6 illustrates the main part of the distillation apparatus shown in FIG. 5, and describes the n-th distillation column Dn.
The gas from the bottom part of the former distillation column flows through the valve 10 and the gas-feeding line 11 to the top part of the distillation column 12. The gas from the top part of the distillation column 12 flows through the gas line 13 to the condenser 14. This gas is cooled therein by a cooling fluid such as liquefied nitrogen from the line 15, and then is liquefied and flows to the liquid line 16. A part of liquid flowing in the liquid line 16 flows into the liquid-return line 17, while the remainder thereof flows into the liquid-reflux line 18 and is fed to the top part of the distillation column 12 as a reflux liquid.
The liquid flowing in the liquid-return line 17 is returned through the valve 19 to the bottom part of the former distillation column. In this case, as shown in FIG. 6, the valve 19 is opened at the time point when a liquid head pressure ΔP1 originating from the difference in height from the midstream of the liquid line 16 to the valve 19 provided on the liquid-return line 17 becomes higher than the pressure difference ΔP2 between the former distillation column and the distillation column 12, i.e. the time point when the liquid-return line 17 and a part of the liquid line 16 are filled with the liquid. Then, this pressure difference is used as driving force so that the liquid is returned to the bottom part of the former distillation column.
However, the pipe of the liquid-return line 17 has a small inner diameter of 3 to 20 mm in order to decrease liquid hold-up. The liquid flowing from the condenser 14 to the liquid-return line 17 is evaporated by heat inleak so as to become a gas, and then this gas may become an upward flow in the liquid-return line 17, or may become bubbles and stored in the line. Therefore, a liquid may become difficult to be stored in the liquid-return line 17, and it may be difficult to obtain the liquid head pressure required for the return to the former distillation column.
Herein, in the conventional distillation columns shown in FIG. 2 to FIG. 5, the distillation column at the recovery part may not exit depending on the yield of the desired component.
[Non-patent Reference 1] YAMAMOTO Hiroshi, “Atomic Energy Chemical Engineering”, pages 10-48, published by the Nikkan Kogyo Shimbun, Ltd., 1976
[Non-patent Reference 2] KANBE Takashi, et al., “Taiyo Nippon Sanso Technical Report” (23) 20-25 (2004)
[Non-patent Reference 3] KIHARA Hitoshi, et al., “Taiyo Nippon Sanso Technical Report” (23) 14-19 (2004)